For various applications utilizing rubber which require high strength and abrasion resistance, particularly applications such as tires and various industrial products, sulfur cured rubber is utilized which contains substantial amounts of reinforcing fillers. Carbon black is commonly used for such purpose and normally provides or enhances good physical properties for the sulfur cured rubber. Particulate, precipitated silica is also sometimes used for such purpose, particularly when the silica is used in conjunction with a coupling agent. In some cases, a combination of silica and carbon black is utilized for reinforcing fillers for various rubber products, including treads for tires.
In some cases alumina has been used for such purpose either alone or in combination with silica. The term "alumina" can be described herein as aluminum oxide, or Al.sub.2 O.sub.3. Use of alumina in rubber compositions, can be shown, for example, in U.S. Pat. No. 5,116,886 and European Patent publication EPO 631,982 A2.
It is recognized that alumina can be in various forms, namely, acidic, neutral and basic forms. Generally, it is considered herein that the neutral form may be preferred.
In some cases aluminosilicates might be used for such purpose. The term "aluminosilicates" can be described as natural or synthetic materials where the silicon atoms of a silicon dioxide are partially replaced, or substituted, either naturally or synthetically, by aluminum atoms. For example, about 5 to about 90, alternatively about 10 to about 80 percent of silicon atoms of a silicon dioxide might be replaced, or substituted, naturally or synthetically, by aluminum atoms to yield an aluminosilicate. A suitable process for such preparation might be described, for example, as by a coprecipitation by pH adjustment of a basic solution, or mixture, of silicate and aluminate also, for example, by a chemical reaction between SiO.sub.2, or silanols on the surface of a silicon dioxide, and NaAlO.sub.2. For example, in such coprecipitation process, the synthetic coprecipitated aluminosilicate may have about 5 to about 95 of its surface composed of silica moieties and, correspondingly, about 95 to about 5 percent of its surface composed of aluminum moieties.
Examples of natural aluminosilicates are, for example, Muscovite, Beryl, Dichroite, Sepiolite and Kaolinire. Examples of synthetic aluminosilicates are, for example, Zeolite and those which might be represented by formulas such as, for example, [(Al.sub.2 O.sub.3).sub.x.(SiO.sub.2).sub.y.(H.sub.2 O).sub.z ]; [(Al.sub.2 O.sub.3).sub.x.(SiO.sub.2).sub.y.MO]; where M is magnesium or calcium. Use of aluminosilicates in rubber compositions, can be shown, for example, in U.S. Pat. No. 5,116,886, European Patent publication EPO 063,982 A2, Rubber Chem. Tech., Volume 50, page 606 (1988) and Volume 60, page 84 (1983).
It is important to appreciate that, conventionally, carbon black is a considerably more effective reinforcing filler for rubber products, and particularly for rubber tire treads than silica unless the silica is used in conjunction with a coupling agent, which may sometimes be referred to as a silica coupler or silica adhesive compound or coupling agent.
Such silica coupler or silica adhesive agent may, for example, be premixed, or pre-reacted, with the silica particles or added to the rubber mix during the rubber/silica processing, or mixing, stage. If the coupling agent and silica are added separately to the rubber mix during the rubber/silica mixing, or processing stage, it is considered that the coupling agent then combines in situ with the silica.
In particular, such coupling agents are sometimes composed of an organosilane such as an organosilane polysulfide which has a constituent component, or moiety, (the silane portion) capable of reacting with the silica surface and, also, a constituent component, or moiety, capable of reacting with the rubber, particularly a sulfur vulcanizable rubber which contains carbon-to-carbon double bonds, or unsaturation. In this manner, then, the coupler acts as a connecting bridge between the silica and the rubber and thereby enhances the rubber reinforcement aspect of the silica.
Numerous coupling agents are taught for use in combining silica and rubber, such as, for example, silane coupling agents containing a polysulfide component, or structure in which the polysulfide bridge portion may be composed of from 2 to 8 sulfur units, such as, for example, an organosilane polysulfide sometimes referred to as bis-(3-triethoxysilylpropyl)tetrasulfide, available from Degussa GmbH, for example, as Si69. It is understood that the sulfur bridge portions of such "tetrasulfide", while having an average of about 3.5 to about 4 connecting sulfur atoms, actually has from about 2 to about 6 or 8 connecting sulfur atoms in its bridge portions where not more than 25 percent of its bridge portions contain two connecting sulfur atoms. Therefore, it is considered herein that at least 75 percent of its sulfur bridge portions contain 3 or more connecting sulfur atoms. For example, see U.S. Pat. Nos 4,076,550; 4,704,414; and 3,873,489.
It is recognized that such organosilane polysulfides which contain 3 or more connecting sulfur atoms in their sulfur bridges can also act as a sulfur donor for the liberation of free sulfur to participate in a vulcanization, or partial vulcanization, of a sulfur vulcanizable elastomer since free sulfur may be liberated therefrom at a temperature of, for example, about 150.degree. C. above. It is considered herein that such recited temperature is approximate in nature and is dependent upon a choice of various individual organosilane polysulfides as well as other factors, although it is believed that at temperatures lower than about 150.degree. C., for most practical organosilane polysulfides which contain from 3 to 8 sulfur atoms in their sulfur bridge portions, the liberation of free sulfur, if any, occurs at a relatively slow rate.
Such temperatures may be experienced, for example, in preparatory, or what is often called non-productive, mixing step for blending rubber and rubber compounding ingredients, typically exclusive of addition of free sulfur, sulfur donors and/or rubber vulcanization accelerators. Such mixing might typically occur, for example, at a temperature in a range of up to about 140.degree. C. to about 180.degree. C.; and most likely at least a portion of the mixing occurs at a temperature of at least 160.degree. C. or above. The small amount of free, liberated, sulfur is then available to combine with and/or possibly partially vulcanize, the unsaturated elastomer with which the silica and coupler are being mixed in such mixing stages.
Bis-(3-triethoxysilylpropyl) disulfide, as a variety of organosilane polysulfide, is also taught as being useful as a silica coupler for a silica-containing sulfur vulcanizable elastomer composition, even as a high purity form of such disulfide in, for example, U.S. Pat. No. 4,046,550 and German Patent Publication DT 2,360,471. However, it is considered herein that such disulfide does not ordinarily liberate free sulfur in such aforementioned rubber/silica/coupler mixing operation.
In practice, sulfur vulcanized elastomer products are typically prepared by thermomechanically mixing rubber and various ingredients in a sequentially stepwise manner followed by shaping and curing the compounded rubber to form a vulcanized product.
First, for the aforesaid mixing of the rubber and various ingredients, typically exclusive of sulfur and sulfur vulcanization accelerators, the elastomer(s) and various rubber compounding ingredients are typically blended in at least one, and usually at least two, preparatory thermomechanical mixing stage(s) in suitable mixers. Such preparatory mixing is often referred to as non-productive mixing, or non-productive mixing steps or stages. Such preparatory mixing is usually conducted at temperatures up to about 140.degree. C. to 190.degree. C. and often up to about 150.degree. C. to 180.degree. C.
Subsequent to such preparatory mix stages, in a final mixing stage, sometimes referred to as a productive mix stage, sulfur and sulfur vulcanization accelerators, and possibly one or more additional ingredients, are mixed with the rubber compound, or composition, typically at a temperature in a range of about 100.degree. C. to about 130.degree. C., which is a lower temperature than the temperatures utilized in the preparatory mix stages in order to prevent or retard premature curing of the sulfur curable rubber, which is sometimes referred to as scorching, of the rubber composition.
The rubber mixture, sometimes referred to as a rubber compound or composition, is typically allowed to cool, sometimes after or during a process of intermediate mill mixing, between the aforesaid various mixing steps, for example, to a temperature of about 50.degree. C. or lower.
Such sequential non-productive mixing steps, including the intermediary mill mixing steps and the concluding final mixing step are well known to those in the rubber mixing art.
By thermomechanical mixing, it is meant that the rubber compound, or composition of rubber and rubber compounding ingredients, is mixed in a rubber mixture under high shear conditions where it autogeneously heats up, with an accompanying temperature rise, as a result of the mixing primarily due to shear and associated friction within the rubber mixture in the rubber mixer.
Such thermomechanical rubber compound mixing procedure and associated sheer and accompanying temperature rise aspects are well known to those having experience in the rubber preparation and mixing art.
Where an organosilane polysulfide, or organosilicon polysulfide as it might sometimes be referred to, is used as a silica coupler in a silica reinforced, sulfur curable rubber composition, it is typically added in one or more of such preparatory, non-productive, mix stages where, as hereinbefore pointed out, the mixing may typically occur at a temperature up to, for example, about 140.degree. C. to about 190.degree. C. or perhaps up to about 150.degree. C. to about 180.degree. C.
Uniquely, when such hereinbefore described organosilane polysulfide containing three or more connecting, or connected-in-series, bridge sulfur units is utilized and added in an aforesaid preparatory mix stage in which sulfur vulcanizable elastomers, silica and associated compounding ingredients are mixed to a temperature to, for example, about 140.degree. C. to about 180.degree. C., at least three chemical reactions are considered herein to take place.
The first reaction is a relatively fast reaction and is considered herein to take place between the silica and the silane moiety of an organosilane, such as, for example, an organosilane polysulfide. Such reaction may occur at a relatively low temperature such as, for example, at about 120.degree. C. Such reaction is well known to those having experience in the silica reinforcement of sulfur vulcanizable elastomers in which organosilane polysulfides are used as silica coupling agents.
The second and third reactions are considered herein to take place between the polysulfidic part of the organosilane polysulfide, or silica coupling agent, and the sulfur vulcanizable elastomer, which contains carbon-to-carbon double bonds, at a higher temperature; for example, above about 140.degree. C. One or more of such second and third reactions are believed to be well known to those having skill or experience in such art.
The aforesaid second reaction is considered herein to consist in a degree of grafting of the organosilane polysulfide onto the elastomer backbone through a covalent bond between a sulfur of the organosilane polysulfide and a carbon atom of the polymer chain of the elastomer, probably a carbon atom alpha to a carbon-to-carbon double bond in the elastomer. Such a mechanism has been described in the literature (S. Wolff, Rubber Chem. Tech., 55 (1982) 967). This reaction is believed herein to be a key for the reinforcing action of organo silane polysulfide coupling agent in silica reinforced sulfur vulcanizable, or sulfur vulcanized as the case may be, rubber compositions.
The third reaction is considered herein as being dependent upon the organosilane polysulfide as a sulfur donor, or a provider of free sulfur. Because of the nature of the thermal stability of the S.sub.x (where x is 3 or more) bridge of the organosilane polysulfide, the energy and associated resultant temperature, particularly a temperature in a range of about 150.degree. C. to about 180.degree. C., involved in thermomechanical mixing of rubber compositions is sufficient to break the sulfur bridge of organosilanes polysulfides a with a sulfur bridge of three or more connecting sulfur atoms. Thus, a small amount of free sulfur is usually formed. Such small amount of liberated free sulfur is then available to partially vulcanize the elastomer in a normal elastomer vulcanization manner. Such partial vulcanizing, or curing, can be considered as somewhat of a side reaction in a sense that such vulcanization is not considered herein as a direct aspect of coupling the silica to the rubber via the silica coupler, or adhesive agent. Indeed, such pre- or partial vulcanization, or partial crosslinking, of the elastomer by the liberated free sulfur can lead to significant processing difficulties where the resultant viscosity of the rubber composition becomes too high to be suitably processed in typical rubber mixing and/or processing equipment or, in an alternative, the resultant viscosity of the rubber composition may becomes somewhat inconsistent from rubber batch to rubber batch, particularly where the rubber composition mixing time or temperature may incrementally vary.
Accordingly, and in part because of the complexity of the combination of both the aforesaid desirable silane-polymer chemical reaction and, also, the sulfur donor effect of the organosilane polysulfide containing a bridge of three or more connecting sulfur atoms, occurring within the aforesaid rubber composition mixing processes at temperatures in a range of about 140.degree. C. to about 180.degree. C. under high sheer conditions, it has been observed that it is sometimes difficult to obtain a consistent rubber product, from rubber mixture to rubber mixture, as may be evidenced by rubber products with inconsistent physical properties. Accordingly, the complexity of the possible reaction mechanisms makes it very difficult, if not impractical in some cases, to suitably control the overall reaction of the organosilane polysulfide reaction and the processing of the rubber, particularly during the non-productive mixing steps while the aforesaid multiplicity of chemical reactions are taking place.
It is proposed, and as hereinafter set forth has been discovered, that a very different and more controllable sulfur/rubber interaction can be effected where the aforesaid covalent bonding between the silane coupling agent and the elastomer (aforesaid second reaction) on the one hand and the aforesaid sulfur donor effect (aforesaid third reaction) on the other hand are decoupled from each other, at least insofar as the organosilane polysulfide is concerned.
Indeed, such decoupling is considered herein to be an important and significant aspect of the invention.
In order to accomplish the decoupling effect, it is envisioned to limit the organosilane polysulfide to an organosilane polysulfide in which the polysulfide is limited to a disulfide, or at least a relatively high purity disulfide. Such high purity organosilane disulfide would contain minimal, if any, attendant, or accompanying, organosilane polysulfides containing a sulfur bridge of three or more connecting sulfur atoms.
The action of the organosilane disulfide might be explained, for example, in the following way:
First, the disulfide moiety of the organosilane disulfide does not appreciably or readily form free sulfur during the aforesaid preparatory, nonproductive rubber mixing steps at temperatures in a range of 140.degree. C. to 180.degree. C., particularly during the relatively short individual mixing periods in a typical rubber mixing step, or sequential series of, as the case may be, mixing steps, in a rubber mixer or mixers of an overall mixing period, of, for example, less than about 15 minutes or perhaps even 20 or more minutes. This is because, for the organosilane polysulfide, the energy required to break the sulfur-to-sulfur bonds of the disulfidic bridge and/or the associated carbon-to-sulfur bonds adjacent to the disulfidic bridge is much higher than the energy needed to similarly break such bonds for a polysulfidic bridge composed of three or more connecting sulfur atoms in an otherwise similar organosilane polysulfide.
Therefore, it has occurred to the inventors that, in a process of mixing sulfur curable rubber, silica and organosilane polysulfide at elevated temperatures, the reaction involving the formation of a covalent bond between the organosilane polysulfide and the elastomer (aforesaid second reaction) can be decoupled from a sulfur donating effect (aforesaid third reaction) may be accomplished by using such organosilane polysulfide in a form of a relatively high purity disulfide version of the polysulfide in combination with a separate and independent addition of a vulcanization accelerator, or a sulfur source such as, for example, a source of free, or elemental, sulfur or a combination of both vulcanization accelerator and sulfur source during at least one of the aforesaid preparatory rubber composition mixing steps.
Such sulfur source may be, for example, in a form of elemental sulfur, or S.sub.8, itself, or a sulfur donor. A sulfur donor is considered herein as a sulfur containing organic compound which liberates free, or elemental sulfur, at a temperature in a range of about 140.degree. C. to about 190.degree. C. Such sulfur donors may be, for example, although are not limited to, polysulfide vulcanization accelerators and organosilane polysulfides with at least three connecting sulfur atoms in its polysulfide bridge.
The amount of free sulfur source addition to the mixture can be controlled or manipulated as a matter of choice relatively independently from the addition of the aforesaid organosilane disulfide. Thus, for example, the independent addition of sulfur source may be manipulated by the amount of addition thereof and by sequence of addition relative to addition of other ingredients to the rubber mixture such as, for example, the silica reinforcement.
In such manner, then, the organosilane disulfide, with its two connecting sulfur atoms in its sulfur bridge portion, could be utilized for the aforesaid first and second associated reactions and the independent addition of the sulfur source particularly a free sulfur source, could be primarily relied upon for the aforesaid third reaction.
Thus, it is considered herein, that at least a partial decoupling of the aforesaid reactions is postulated.
It is considered herein that such manipulation of the sulfur/rubber interaction, combined with the aforesaid silane/silica interaction in a preparatory rubber mixing step is a significant departure from known prior practice.
It is recognized that prior patent publications recite, or include, the use of organosilane disulfides as silica couplers in elastomer formulations. For example, see U.S. Pat. No. 4,076,550 and German Patent Publication DT 2,360,470 A1. However, it is believed that the inventors' prescribed procedure of utilizing a relatively high purity organosilane disulfide in combination with independently adding a free sulfur source in a preparatory rubber-silica mixing step, particularly as a means of controlling the associated sulfur/elastomer interaction is an inventive departure from prior practice. Such procedure has been observed to provide a better reproducability of rubber compound physical properties as well as considerably greater compounding, or mixing, flexibility as indicated by the controllable reaction effect.
Further to the aforesaid inventive concept, it is also considered herein that an addition of an alkyl silane to the coupling agent system (organosilane disulfide plus additional free sulfur source and/or vulcanization accelerator) typically in a mole ratio of alkyl silane to organosilane disulfide in a range of about 1/50 to about 1/2 promotes an even better control of rubber composition, or compound, processing, and usually resultant compound aging under aging conditions such as exposure to moisture and/or ozone.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber, or elastomer".
In the description of this invention, the terms "rubber" and "elastomer" if used herein, may be used interchangeably, unless otherwise prescribed. The terms such as "rubber composition", "compounded rubber" and "rubber compound", if used herein, are used interchangeably to refer to rubber which has been blended or mixed with various ingredients and materials and "rubber compounding" or "compounding" may be used to refer to the mixing of such materials. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
A reference to an elastomer's Tg, if used herein, refers to a glass transition temperature which can be determined by a differential scanning calorimeter at a heating rate of 10.degree. C. per minute.